About the Reactance Calculator
Capacitors and inductors don't resist AC current the way a plain resistor does. Their opposition, called reactance, depends on signal frequency, and this calculator works out that frequency-dependent opposition in ohms for either component type. It's built for anyone sizing a filter, crossover, or tuned circuit who needs to know how a capacitor or inductor behaves at a specific frequency.
How It Works
Choose whether you're working with a capacitor or an inductor, enter the AC frequency in hertz, and enter the component's value (capacitance in microfarads or inductance in millihenries, depending on which type you selected). The calculator converts that value into base SI units internally and applies the matching reactance formula to return a result in ohms.
Formula & Methodology
For a capacitor, the calculator multiplies the entered microfarad value by 1e-6 to get farads, then plugs it into Xc = 1/(2 pi f C); because capacitance sits in the denominator, larger capacitors and higher frequencies both push reactance down. For an inductor, the entered millihenry value is multiplied by 1e-3 to get henries and used in XL = 2 pi f L; here inductance and frequency are both in the numerator, so reactance climbs as either one increases. Both branches require frequency and component value to be strictly greater than zero, otherwise the calculation is blocked before it runs.
Examples
Capacitor at mains frequency
A 100 µF capacitor at the standard 60 Hz US mains frequency gives a capacitive reactance of about 26.5 Ω, from 1/(2 x pi x 60 x 0.0001).
Inductor at the same frequency
A 100 mH inductor at that same 60 Hz gives an inductive reactance of about 37.7 Ω, from 2 x pi x 60 x 0.1.
Advantages
- Covers both capacitive and inductive reactance in a single tool instead of requiring two separate calculators.
- Automatically handles the microfarad-to-farad and millihenry-to-henry unit conversions so you can enter component values as printed on the part.
- Gives a quick way to see how reactance shifts as you sweep the frequency field up or down.
Common Mistakes
- Mixing up the two formulas and dividing by frequency for an inductor or multiplying for a capacitor.
- Entering the value in the wrong unit scale (farads instead of microfarads, or henries instead of millihenries), which throws the result off by several orders of magnitude.
- Treating reactance as if it were the same as total impedance in a circuit that also contains resistance.
Edge Cases to Watch For
- A frequency or component value of zero or less returns an error rather than a divide-by-zero or a nonsensical negative reactance.
- At very low frequencies, capacitive reactance grows extremely large, which is mathematically correct but can produce results in the megaohm range that are easy to misread.
- Switching component type without also rechecking the value's unit (µF versus mH) is a frequent source of wrong answers, since the field's meaning changes with the selected type.
Common Use Cases
- Audio hobbyists sizing crossover capacitors or inductors for speaker filters.
- Electronics students working through AC circuit homework that asks for Xc or XL at a given frequency.
- Power supply and filter designers estimating how a component will behave at their target switching frequency.